This invention relates to the separation and purification of C-4 hydrocarbons, particularly butadiene-1,3, from C-4 hydrocarbon mixtures utilizing extractive distillation with a selective solvent. It pertains to such extraction distillation when using an alkoxynitrile as a selective solvent component and is specifically directed to modifying the solvent composition as will be hereinafter described.
It is well known to the art that C-4 hydrocarbon mixtures containing C-4 hydrocarbons of different degrees of unsaturation, such as mixtures of butanes and butylenes or mixtures of butadiene-1,3 with butanes (including n-butane and isobutane) and butylenes (including butene-1, and cis- and trans-butene-2) which are not easily separable by ordinary fractional distillation because of similarities in boiling points and azeotrope formation are much more efficiently separated into their individual components by the process of extractive distillation with a solvent of relatively higher boiling point which selectively dissolves one or more of the more-unsaturated components. In the extractive distillation process as conventionally carried out, the selective solvent is introduced near the top of a distillation column and flows down the column as the distillation proceeds where it is contacted with the vapors of the hydrocarbons as they travel up the column. The more saturated hydrocarbons not dissolved by the solvent go overhead while the bottoms of the column contain the solvent plus the more unsaturated hydrocarbon components, which are removed from the solvent in a stripping column, or by other suitable means, and the lean solvent is recirculated to the column.
The choice of the selective solvent for use in the extractive distillation separation of butadiene-1,3 from C-4 hydrocarbon fractions of commercial availability (as from the dehydrogenation of butane or the oxydehydrogenation of butylenes or as a by-product of catalytic cracking of petroleum feedstocks such as naphtha to produce ethylene or propylene) has been the subject of intensive investigation for many years.
Acetone was one of the first proposed selective solvents but the first commercially practiced extractive distillation processes for separation of butadiene-1,3 from C-4 hydrocarbon streams used furfural, or a mixture of furfural with a smaller amount of water, as the selective solvent. Processes using other solvents for this purpose were subsequently developed; some of such other solvents of particular prominence include acetonitrile, (ACN), dimethyl formamide (DMF), dimethyl acetamide (DMA), and N-methyl pyrrolidone (NMP). The use of these solvents in place of furfural offered advantages in terms of selectivity but, unfortunately, they could not be substituted in commercial plants already on stream designed for use with furfural without extensive costly modification related to their different physical properties and lack of compatibility with the installed processing scheme.
This disadvantage in the commercial use in existing plants of selective solvents other than furfural is not so significant in the case of the use of an alkoxynitrile solvent, particularly 3-methoxy propionitrile (also known as beta-methoxy propionitrile) in accordance with U.S. Pat. No. 3,372,109. As disclosed therein 3-methoxy propionitrile (MOP) offers advantages over furfural in that it is less corrosive than furfural, more soluble in water which permits the use of greater quantities of water as co-solvent and more selective to the separation of butadiene-1,3 from butylenes.
However, as disclosed in U.S. Pat. No. 3,898,135, attempted practice on a commercial scale in a butadiene plant of the extractive distillation process using as the selective solvent MOP, or a mixture of MOP with water as co-solvent (as a substitute for the use of furfural or aqueous furfural) results in such serious disadvantages as to be entirely unpracticable. Fouling and plugging of equipment due to butadiene-1,3 polymerization are severe resulting in frequent shut downs of operations as well as losses of solvent and of butadiene-1,3 recoverable. The aforesaid U.S. Pat. No. 3,898,135 proposes to overcome these disadvantages by using a selective solvent composition comprising by weight 70 to 98% of alkoxynitrile, preferably MOP, 0.5 to 25% of furfural and 1 to 20% water.
The problem of preventing the polymerization of butadiene-1,3 during its extractive distillation separation from C-4 hydrocarbons fractions using selective solvents other than MOP, specifically DMF, DMA, NMP, ACN, and acetone, had previously been recognized in U.S. Pat. Nos. 3,309,412 and 3,551,507 and proposed therein to be ameliorated by including a small amount of furfural or benzaldehyde or any of a wide range of aromatic nitro compounds, including nitrophenol and dinitrophenol, in the solvent composition. Further, U.S. Pat. No. 3,681,202, directed to a means of effecting improved control and greater capacity in an extractive distillation tower for purification of C-4 or C-5 hydrocarbon streams by the expedient of removal of a side stream and premixing the side stream with the solvent externally of the tower, had previously disclosed that mixtures of acetone, ACN, dimethyl sulfoxide, furfural, NMP, methyl ethylketone, DMA, and MOP, with each other and with water, could be used as solvents for the extractive distillation. Despite these previous disclosures, however, the only known commercial process for the extractive distillation separation of butadiene-1,3 from C-4 hydrocarbon streams using MOP (or other alkoxynitrile) as a selective solvent component remains the process of the aforementioned U.S. Pat. No. 3,898,135 using as the selective solvent a mixture of MOP (or other alkoxynitrile) furfural and water.
The separation and purification of butadiene-1,3 from C-4 hydrocarbon streams using extractive distillation with a selective solvent is at best an energy intensive unit operation because of the necessity to use a large excess of solvent to facilitate the desired separation. Furthermore, the erection of new facilities for production of butadiene-1,3 of the high purity required for polymerization to synthetic rubbers (eg SBR, nitrile rubbers, cis-polybutadiene etc.) is not only quite costly but also complicated, and often impossible, due to environmental considerations. Consequently, there is an urgent need for a butadiene extractive purification process which can be used in existing plants designed for operation with a furfural containing solvent, in which the solvent composition used is more selective for butadiene-1,3 separation than furfural, in which the capacity of the extractive distillation unit to process the C-4 stream is increased, in which there are minimal difficulties due to polymerization of butadiene-1,3 to form a polymer which causes plugging and fouling of equipment, and in which the solvent composition used is completely furfural-free, thereby eliminating the formation of polymers from furfural (whether furfural--butadiene-1,3 adducts or other furfural polymers) and attendant loss of solvent in the "solvent clean-up" operation. In other words, modification of the process of U.S. Pat. No. 3,898,135 by use of a selective solvent composition which improves upon the selectivity of MOP (or other alkoxynitrile) but which overcomes the disadvantages due to the presence of furfural and enables the process to be more efficient and energy saving, is the specific objective of this invention as reflected by its background as hereinabove set forth.